As a flow meter with excellent responsiveness of when measuring the volumetric flow of a measurement object fluid, there is the doppler type ultrasonic flow meter. The principle by which a doppler type ultrasonic flow meter measures the volumetric flow of a measurement object fluid will be explained with reference to FIG. 1.
First, with an oscillator not shown in the drawings a predetermined frequency (basic frequency) f0 is generated, and the frequency of this basic frequency f0 is passed through an emitter (not shown). A pulse electrical signal of the generated basic frequency f0 is inputted to a transmitter 121. As a result of the pulse electrical signal being applied, a pulse electrical signal of the basic frequency f0 is outputted from the transmitter 121 to the inside of a pipe 102 with a measurement object fluid flowing through it. The pulse electrical signal is converted into an ultrasound pulse by an ultrasonic transducer 103, and radiated along a measurement line ML to inside the pipe 102.
The radiated ultrasound pulse is reflected by a tracer (reflector) such as air bubbles in the pipe 102. The reflected ultrasound echo is received by the ultrasonic transducer 103.
The received ultrasound echo is converted into an echo electrical signal by the ultrasonic transducer 103. The converted echo electrical signal is amplified by an amplifier not shown in FIG. 1, and converted into a digital echo signal by an A/D convertor 122. The digital echo signal is inputted to a flowrate calculating circuit 123.
In the flowrate calculating circuit 123, the electrical signal of basic frequency f0 radiated into the pipe 102 from the transmitter 121 and the digital echo signal obtained from the reflected wave are compared. The frequency of the digital echo signal is shifted as a result of it passing through the measurement object fluid flowing at speed inside the pipe 102. And the flowrate of the measurement object fluid is calculated from the frequency difference between the two signals.
In a flow profile calculating circuit 124, a flow profile in a reflected wave measurement region on the measurement line ML is obtained. The obtained flow profile is corrected with the angle of incidence α of the ultrasound to obtain a flow profile in a cross-section perpendicular to the center axis of the pipe 102. The volumetric flow at a given time can be obtained by integrating the obtained flow profile over the cross-sectional area perpendicular to the center axis of the pipe 102.
In the vicinity of the wall of the pipe 102 on the side where the ultrasonic transducer 103 is disposed a flow profile is not obtained correctly. This is because the ultrasound emitted from the ultrasonic device in the ultrasonic transducer 103 is reflected by the end face of the ultrasonic transducer 103 and the inner wall face of the pipe 102 and becomes a large noise with respect to the proper ultrasound signal scattered from particles inside the pipe 102, and a doppler signal cannot be correctly obtained.
As a doppler type ultrasonic flow meter that solves this problem, a doppler type ultrasonic flow meter with which it is possible to display a flow profile under the premise that in relation to the axial-direction center of the pipe the flow on one side is symmetrical with the flow on the other side has been disclosed (for example patent Document 1). FIG. 2 is an example of this display. For the flowrate near the pipe wall of the pipe where noise is large because the ultrasonic transducer is disposed there, a flow profile is obtained under the premise that it is symmetrical about the axial-direction center of the pipe with the flowrate on the side where the noise is relatively small, and displayed on a monitor or the like.
As another doppler type ultrasonic flow meter that solves the above-mentioned problem, a doppler type ultrasonic flow meter which, for the flow profile on the side near the pipe wall where noise is large, extrapolates flow profile data of the pipe wall side where the noise is small has been disclosed (for example patent Document 2). FIG. 3 shows a flow profile and a corrected flow profile detected by a doppler type ultrasonic flow meter of patent Document 2. The flow profile (region Xn) where the noise is large showing the flow profile of FIG. 3 (A) is not used in the calculation of the volumetric flow. Its value is corrected by extrapolating (C) of the same figure. For the region where the noise is small (region Xm), (B) is assumed to be a flow profile with a normal value and is used unchanged in the calculation of the volumetric flow. By this method it is possible to compensate for the fall in measurement accuracy near the pipe wall on the side where the ultrasonic transducer is mounted.
Patent Document 1: JP-A-2004-12204
Patent Document 2: JP-A-10-281832
The prior art mentioned above can only be used in cases where it can be assumed that the flow profile is symmetrical on the side where the ultrasonic transducer is mounted and the opposite side. And, it cannot be applied in locations where the flow of the measurement object fluid curves or in locations where flows converge.